docs: def get_billing_items(self, identifier):
          """Gets all topLevelBillingItems from a specific invoice
  
          :param int identifier: Invoice Id
          :return: Billing_Invoice_Item
          """
  
          mask = """mask[
              id, description, hostName, domainName, oneTimeAfterTaxAmount, recurringAfterTaxAmount, createDate,
              categoryCode,
              category[name],
              location[name],
              children[id, category[name], description, oneTimeAfterTaxAmount, recurringAfterTaxAmount]
          ]"""
          return self.client.call(
              'Billing_Invoice',
              'getInvoiceTopLevelItems',
              id=identifier,
              mask=mask,
              iter=True,
              limit=100
          )